The present invention relates generally to MR imaging and, more particularly, to a method of scaling MR spectroscopic data acquired with a phased-array or surface coil arrangement. Moreover, the present invention includes a system that supports the acquisition of MR spectroscopic data for quantitative analysis of proton single voxel (volume element) spectra with a receive coil arrangement that also supports the acquisition of MR imaging data.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR spectroscopy (MRS) is a common MR technique used for the determination of individual chemical compounds or metabolites located within a volume of interest. The underlying principle of MRS is that atomic nuclei are surrounded by a cloud of electrons which slightly shield the nucleus from any external magnetic field. As the structure of the electron cloud is specific to an individual molecule or compound, the magnitude of this screening effect is then also a characteristic of the chemical environment of individual nuclei. Since the resonance frequency of the nuclei is proportional to the magnetic field it experiences, the resonance frequency can be determined not only by the external applied field, but also by the small field shift generated by the electron cloud. Detection of this chemical shift, which is usually expressed as “parts per million” (ppm) of the main frequency, requires high levels of homogeneity of the main magnetic field B0.
Quantitative analysis of proton single voxel spectra in terms of arbitrary units has become a widely-used and standard approach for carrying out MRS exams. Typically, the quantitative analysis relies or is predicated upon the direct proportionality of acquired MRS signal and metabolite concentrations in the voxel when the MRS signal is scaled by transmitter and receiver gains. This proportionality is particularly accurate for RF signal transmission and reception with linear or quadrature volume resonators or coils. However, such scaling can be inaccurate when the MRS signal is acquired with surface or phased-array coils as a result of the spatially varying B1 sensitivity of these coils.
Phased-array coils are frequently being used to acquire MR imaging data, but not MRS data. MRS data is generally acquired with volume resonators characterized by spatially homogeneous B1 sensitivity. However, a number of imaging techniques, including parallel imaging, require phased-array coil arrangements. In addition, other imaging techniques prefer phased-array coil arrangements because these arrangements advantageously combine the preferred SNR characteristics of smaller FOV coils with the extended FOV coverage afforded by larger volume resonators. As noted above, however, it is impractical to use phased-array coils to acquire MRS data in order to carry out a quantitative analysis of proton single voxel spectra. To conduct an MR imaging scan with parallel imaging techniques as well as acquire MRS data for quantitative analysis requires a change between phased-array coils and volume resonators during the clinical exam—a time consuming, arduous, and impractical task.
Other proposed approaches to resealing MRS data acquired with phased-array or surface coils utilize coil sensitivity maps. Generation of coil sensitivity maps, however, requires separate imaging scans—a scan to acquire the MRS data and a scan to acquire coil sensitivity data. Separate scans decrease patient throughput and are therefore not a viable solution.
It would therefore be desirable to have a system and method capable of quantitative MRS analysis of MR spectroscopic data acquired with a phased-array or surface coil arrangement.